1. Field of the Invention
The present invention relates to a liquid crystal display device, more specifically, a liquid crystal display device in which gray scale display is made by both the voltage gray scale method and the time ratio gray scale.
2. Description of the Related Art
A technique that has recently accomplished rapid development is to manufacture a semiconductor device in which semiconductor thin films are formed on an inexpensive glass substrate, for example, a thin film transistor (TFT). This rapid development is caused by a growing demand for active matrix type liquid crystal display devices.
In an active matrix liquid crystal display device, a pixel TFT is placed in each of pixel regions as many as several hundred thousands to several millions arranged in matrix, and electric charge that flows into and out of a pixel electrode connected to each pixel TFT is controlled by the switching function of the pixel TFT.
As images are displayed with higher definition and higher resolution, demand for multi-gray scale display, desirably, in full color, has been established in recent years.
Recently an active matrix liquid crystal display devices are often used, not only for display devices of notebook type personal computers, but also for display devices of desk-top type personal computers.
In case of using an active matrix type liquid crystal display device comprising a digital driver for the display device of a personal computer, digital video data outputted from the personal computer can be inputted directly into the active matrix liquid crystal display device.
An active matrix liquid crystal display device comprising a digital driver is superior in interface with personal computers etc., which output digital video data as described above. However, active matrix liquid crystal display devices having digital drivers have not yet reached the stage of being introduced into the market as products, since they have complicated circuits which results in large driver area.
Accordingly liquid crystal display devices having analog drivers are frequently used. This is because an active matrix liquid crystal display device having analog driver is simple in its driver construction, and has high yield.
Yet it is necessary to input digital video data outputted from a personal computer into an active matrix liquid crystal display device after converting the digital video data into analog video data by D/A converter circuits when using an active matrix liquid crystal display device for the display device of the personal computer.
There are various kinds of D/A converter circuits (DAC) which convert digital video data inputted from the external into analog data (gray scale voltage).
Multi-gray scale display capacity of an active matrix display device is dependent on how many bits of digital video data the D/A converter circuit can convert into analogue data. For instance in general, a display device having a D/A converter circuit that processes 2 bit digital video data is capable of 22=4 gray scale display. If the circuit processes 8 bit data, the device is capable of 28=256 gray scale display, if n bit, 2n gray scale display.
However the circuit construction of D/A converter circuits become complicated in order to enhance the capacity of D/A converter circuits, which costs high even incases of loading the D/A converter circuits in the external of an active matrix liquid crystal display device.
Further, a problem arose in the response speed of liquid crystal molecules in a conventionally well-known TN mode (twist nematic mode) which uses nematic liquid crystal, as the time for writing an image data onto a pixel became shorter, due to large sized display, high precision and high resolution of an active matrix liquid crystal display device.
As described above, materialization of an active matrix liquid crystal display device which achieves large sized display, high precision, high resolution and multi gray scale has been desired.